1. Field of the Invention
This invention relates to a semiconductor laser device and an optical pickup device using the semiconductor laser device, especially to a semiconductor laser device and an optical pickup device using the semiconductor laser device that uses a fewer number of the parts resulting in a lower cost.
2. Description of the Related Art
An optical disc device, which can perform reading action and recording action of signals by emitting laser light from an optical pickup device to a signal recording layer of an optical recording media (optical disc), has been widely known.
Efforts have been made to make a smaller and lighter optical pickup device that can accommodate multiple kinds of optical discs (such as CD (compact disc) or DVD (digital versatile disc)) smaller and lighter. For example, an object lens which is shared among laser lights with different wavelength is used. A laser device that has a plurality of semiconductor laser diodes (LD, hereinafter) with different wavelengths as one package is also known for using in the optical pickup device.
The optical system of the optical pickup device can be simplified and the number of parts used in the device can be lowered, leading to the lower cost, by putting a plurality of LDs with different wavelengths in one laser device as one package and by using one optical system with one lens for plurality of laser lights with different wavelengths (see the first patent document).
A device using an optical disc with improved recording density, such as blue-ray disc or HD-DVD (high density digital versatile disc), has recently been developed. An optical disc device and an optical pickup device that can be shared by these discs with the improved density has been highly demanded.
The light source for this kind of optical pickup device requires a LD that can emit laser lights with three different wavelengths, for example, one for Blue-ray Disc (BD), one for DVD, and one for CD. Therefore, a three-wavelength LD, which has these LDs in one package, has been developed. This technology is described for instance in Japanese Patent Application Publication No. 2002-163837.
FIG. 9 shows how one object lens converges laser lights from a three-wavelength LD 510. FIG. 9 is a simplified diagram showing the relationship in paraxial between laser lights emitted from each of the LDs and one optical system with one object lens for converging the lights in a three-wavelength LD 510 with three LDs (LD 501 for BD, LD 502 for DVD, and LD 503 for CD).
The LDs 501, 502, and 503 are disposed side by side on a package supporting substrate 511 and three laser lights are converged by one object lens 505. The light coming from each of the LDs goes to the center of a collimator 504. The light becomes a parallel light at the collimator 504 and goes into the lens 505. When the starting locations (the distance among the light emission points E1′, E2′, and E3) of the laser lights with three-wavelengths from each of the LDs are away from each other, laser lights are shifted away from the optical axis of the collimator 504, leading to the incline θ1 and θ2 in the lights coming from the collimator 504. Then, aberration takes place when the light comes into the object lens 505 depending on the amount of incline (height of the image).
For example, when the distance L1′ between each of the lights E1′, E2′, and E3′ and the collimator 504 is 15 mm, and the distance L2′ between the light E1′ and E2′ is 0.3 mm, the incline θ1 of the light (shown as a solid line in the figure) coming from the collimator 504 of the LD 501 used for BD will be 1.2 degrees, leading to aberration of 0.090λ depending on the design of the object lens. It will be a factor for the deteriorated function of the optical pickup device.
Therefore, it is necessary to dispose three light emission points E1′, E2′, and E3′ close to each other in the three-wavelength LD. However, it is difficult to put LDs with three different wavelengths closer when they are disposed side by side on the supporting substrate 511. Since the mounting error is about ±20 μm in general and the width W of the each LD is between 200 μm and 300 μm, it is very difficult to reduce the distance (L2) among the light emission points E1′, E2,′ and E3′.
FIG. 10 is another figure showing the three-wavelength LD 520. It is a side view of the three-wavelength LD 520 viewing from y axis, from which laser light comes from.
The LD for DVD and the LD for CD can be integrated on the same semiconductor substrate. Therefore, it is possible to form the three-wavelength LD 520 in which the LD 501 for BD is mounted on a monolithic chip with the LD 506 for both DVD and CD, as shown in FIG. 10.
The emission point E2′ of the LD for DVD and the emission point E3′ of the LD for CD can be formed through photo lithography in the LD 506 for both DVD and CD. Thus, the mounting error can be reduced in this configuration compared to the case that they are mounted independently (FIG. 9).
The emission point E1′ of the LD 501 for BD can be aligned with one of the emission points (for example, E2′) of the LD 506 for DVD and CD when the LD 501 for BD is mounted on the LD 506 for DVD and CD. In this configuration, only the distance L3′ between two emission points E2′ and E3′ in the LD 506 for DVD and CD need to be considered.
However, exothermic property is not ideal in the layered configuration described above, especially that of the LD on the top layer (for example, the LD 501 for BD), deteriorating the reliability of the device. High exothermic property is required for the LD 501 for BD, as it generates large amount of heat.
Also, the semiconductor substrate (GaAs substrate) using LD 506 for DVD and CD does not provide a good exothermic property when the LD 506 for DVD and CD is disposed at the lower layer, which is another factor to deteriorate the reliability of the LD 501 for BD at the upper layer.
It is possible to dispose the LD 501 for BD as a lower layer for the better exothermic property. However, the chip size of the LD 501 needs to be increased. The semiconductor substrate (for example, GaN) for the LD 501 for BD has high cost. Therefore, the increased chip size will lead to a hiked manufacturing cost.